In our U.S. Pat. No. 7,152,001, the entirety of which is herein incorporated, there is disclosed a computer based system for predicting the fluid level in a fluid flow network. The system has been very successful as it can use past and present measurements of parameters to predict and control fluid level and flow. The system gathers data from timed fluid levels and opening positions of regulators or valves to provide a model from which fluid levels and flow can be determined in real time.
In our International Patent Application No. PCT/AU2012/000907, the entirety of which is herein incorporated, there is disclosed a method of demand management for fluid networks. The method was applicable to both a closed conduits (pipeline network) and open conduits (channel networks), Gravity pipe networks typically operate within limited pressure head and therefore are constrained in their capability to meet demand.
Known models for pipe networks would be used in the management of demand for these networks. Data from the SCADA system would be used to calibrate and continually fine-tune the model of the pipe conveyance network based on system identification techniques. Flow measurement and pressure head measurements would be located at points on the pipe network that would be deemed necessary to calibrate the model to the desired accuracy. The supply points to users are the primary form of control used with a pipe network. The controller for a pipe network is much simpler than it is for a channel network with the principle form of control being maintaining the flow at the supply point equal to that of the order.
Control and management of demand is especially applicable to gravity pipe networks commonly used for the supply of irrigation water. Difficulties have arisen to implement such systems as gravity pipe networks typically operate within limited pressure head and therefore are constrained in their capability to continually meet demand. Gravity pipelines also typically operate at lower pressure heads where there will be greater interaction between flows at outlets due to valve operations. Accordingly, assuming all the parameters such as pipe diameter, flow rate, valve size, etc being the same, the higher the static pressure head (e.g. from pumping), the less sensitive the impact of flow fluctuations due to valve operations (e.g. valves opening or closing) on other valves in operation.
FIG. 1 illustrates why operating valves are less sensitive to flow variations in the supply pipeline (e.g. from other opening and closing valves) with a higher pressure head in the pipeline. FIG. 1 shows a graph of the hydraulic grade line or pressure head against the valve position for a high pressure at line 10 and for a low pressure or gravity fed hydraulic grade line or pressure head at line 12. Gravity fed pipe 14 is shown on a grade with two valves 16 and 18. Although pipe 14 is shown on a grade, it could be horizontal if the water supply is elevated to provide the required pressure head. For line 10, pipe 14 would be coupled to a pump (not shown) to produce a high-pressure head. The explanation now follows:                1. Assume the one physical pipeline 14 operating at either a Low Pressure (LP) state and at a High Pressure (HP) state, and for a specific operating valve supplying fluid off the pipeline 14.        2. Assume initially the supply pipeline 14 is operating at the same flow rate Q1 in both states.        3. A change in flow in the supply pipeline 14 (due to other valves 16, 18 starting and stopping) occurs for both states.ΔQ=Q1−Q2         4. The change in pressure head, Δh, at the operating valve 16 due to the change in flow ΔQ, is the same for both states. (The known pipeline flow versus pressure head equations, e.g. Colebrook-White equation, Manning's Formulae are applicable)        5. The head loss across valve 16 is determined as follows;        
  h  =      K    ⁡          (                        v          2                          2          ⁢          g                    )                       where                    h=pressure loss in terms of fluid head, i.e. fluid head loss            K=the valve ‘K’ factor (assume constant) for the specified valve opening            v=velocity of fluid            g=acceleration due to gravity                        6. Assume the same initial flow, and therefore velocity, through the operating valve 16 in both the LP and HP states are equal        
            v              LP        ⁢                                  ⁢        1              =          v              HP        ⁢                                  ⁢        1                                h                  LP          ⁢                                          ⁢          1                            K        LP              =                  h                  HP          ⁢                                          ⁢          1                            K        HP                            7. With hLP1<<hHP1 KLP<<KHP          where KLP and KHP represent the different K factors for the different valve openings in either pressure state i.e. valve 16 will be at a greater opening in the LP state than the HP state.        8. When a pressure head change, Δh, is introduced, the change in pressure head across valve 16 for each state hLp2=hLP1−Δh, and hHP2=hHP2−Δh respectively. The relative change in head across the valve is greatest in LP state than the RP state,        9, Assuming valve 16 remains in the same opening position for each state, and therefore the K factors remain the same, the new velocity for each state isvLP2=√{square root over ((hLP1−Δh)2g/KLP)}vHP2=√{square root over ((hHP1−Δh)2g/KHP)}        10 The resulting velocities for each state due to the pressure head change, Δh, will see;(vLP2−vLP1)>>(vHP2−vHP1)         The change in velocity, and therefore flow through the valve is much greater for the LP state than for the HP state.        
Higher pressure (e.g. pumped) pipelines with smaller diameter valves and flow meters have less interaction between operating valves than low-pressure pipelines with larger diameter valves and flow meters. In high pressure systems the valves can be manually positioned to a set opening to achieve a certain flow, and the flow will not be impacted significantly by the operation of the other valves (e.g. valves opening or closing) in the pipeline. Whereas, low-pressure pipelines require an integrated control and demand management system to manage the valve interaction within the tight hydraulic grade line conditions.
FIG. 2 shows pipeline 14 separated from FIG. 1 and illustrates the maximum supply pressure 22 which must maintain the pipeline full to ensure the accuracy of the flow meters (not shown) associated with valves 16, 18, and 20. It is important to keep the pipeline full to make the control problem simple and tractable as a “pipe not full” scenario will significantly change the physics that governs the dynamics of pipe flow. Pipe flow transitioning between “pipe full” and “pipe not full” states will make achieving robust control intractable. Maintaining the hydraulic grade line 12 associated with the pipeline 14 above the maximum supply pressure 22 will also ensure that the pressure head at the valves 16, 18 and 20 are high enough to guarantee the flow rate the valves were designed for. The low-pressure head or hydraulic grade line 12 associated with pipeline 14 will potentially result in increased controller interaction between the discrete control actions necessary to maintain desired flows at the valves. This is further compounded with gravity pipelines where the flow capacity at the valves is high in relation to the overall flow capacity of the main trunk pipeline The action of opening or closing valves will impact the pressure head, and therefore flow, at all other valves on the pipeline 14 that are operating. Therefore there will be interaction between various automated valves operating off the pipeline. In this low energy pipeline, the control will be subject to instability. Each movement in a valve has a level of interaction with all the other operating valves plus supply level variation at the source or at the outlet (on-farm). Because of the low pressure in the pipeline 14, the hydraulic grade line 12 is very sensitive to the operation of the valve/outlets.
This sensitivity is illustrated in FIG. 3 where a graph of flow and time is shown. Line 24 illustrates valve 16 being already open and the effect that the opening of valve 18 has on the network. Line 26 illustrates the flow of valve 18. Both valves 16 and 18 are trying to maintain their preselected flow rate but the valves produce an unstable jittery interaction between the valves. The interaction is fairly minor on the flow through valve 16 shown by the changes in flow at 28 but there is a major interaction on the stability of flow through valve 18 shown by the changes in flow at 30. Furthermore, all the additional valves e.g. valve 20 will also be effected by this interaction. The network becomes extremely unstable and this is a key reason why gravity feed irrigation systems have found little favour with water suppliers and users.